The present invention relates generally to packaging methods and fabrication techniques for making electrochemical cells and multi-cell batteries. In particular, the invention relates to electrochemical cell constructions useful for primary and rechargeable bipolar battery structures that have a high energy storage capacity and efficient battery performance. More specifically, this invention relates to electrochemical cells including positive and negative electrode structures and methods of making such cells that are capable of being stacked in a multi-cell battery construction.
Multi-cell batteries are typically constructed in a broad range of electrochemical systems and are often packaged in cylindrical or prismatic housings. Individual cells are connected in series by conductive links to make the multi-cell batteries. Such construction approaches provide for good sealing of the individual cell compartments and for reliable operation. However, such constructions allocate a large fraction of the multi-cell battery""s weight and volume to the packaging and, thus, do not make full use of the energy storage capability of the active components of the cell. For improving battery energy storage capacity on a weight and volume basis, packaging approaches are sought that reduce packaging weight and volume and that provide stable battery performance and low internal resistance.
These objectives have led to the pursuit of a bipolar construction in which an electrically conductive bipolar layer serves as the electrical interconnection between adjacent cells, as well as a partition between the cells. In this type of construction, the current flows perpendicular from cell to cell over the entire cell area thus increasing high rate capability. However, in order for the bipolar construction to be successfully utilized, the bipolar layer should be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell""s environment, capable of making and maintaining good electrical contact to the electrodes, and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell. These features are more difficult to achieve in rechargeable batteries due to the charging potential that can accelerate corrosion of the bipolar layer and in alkaline batteries due to the creep nature of the electrolyte. Achieving the proper combination of these characteristics has proven to be very difficult.
For maintenance-free operation, it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that may be structurally poor for containment of the gases present or generated during cell operation. In a sealed cell construction, gases are generated during charging that need to be chemically recombined within the cell for stable operation. To minimize weight of the structures used to provide the gas pressure containment, the battery should operate at relatively low pressure. The pressure containment requirement creates additional challenges on designing a stable bipolar configuration.
Also, the need for removal of heat generated during normal operation of batteries may be a limiting design factor in bipolar construction due to the compact nature of the construction. Thus, an optimum bipolar design should provide for removal of heat generated during operation.
In U.S. Pat. No. 5,393,617, electrode structures that are adaptable for primary and electrically rechargeable electrochemical wafer cells are disclosed. According to an embodiment set forth in that patent, a flat wafer cell includes conductive, carbon-filled polymeric outer layers that serve as electrode contacts and as a means of containment of the cell. Multi-cell, high voltage batteries may be constructed by stacking individual cells. Specially formulated electrodes and processing techniques that are compatible with the wafer cell construction are particularly disclosed for a nickel-metal hydride battery system. The cell design and electrode formulation disclosed in the ""617 patent provide for individual operation of a vented or sealed cell and/or for operation of these cells in a stacked array in an outer battery housing.
The foregoing construction approach of the ""617 patent is advantageous and has proven to be flexible for designing batteries having different capacity, voltage and chemistry. However, scientists and engineers working under the direction of Applicant""s assignee are continually seeking to develop further improved wafer cell and battery constructions, and methods of fabrication thereof.
The present invention provides a means for achieving desirable packaging benefits of bipolar construction for multi-cell batteries and of overcoming material and construction problems of some previous approaches. Although the materials of construction for each type of cell are specific to each battery chemistry, the general bipolar construction disclosed herein may be used for many types of electrochemical cells. In particular, several embodiments and examples that follow relate to the rechargeable nickel-metal hydride chemistry but may be generally adaptable to other chemistries.
An advantage of the present invention relates to providing a bipolar battery construction for primary and/or rechargeable multi-cell batteries that have improved energy storage capacity while providing stable and efficient battery performance, as well as long term chemical and physical stability.
Another advantage of the present invention relates to providing a bipolar battery construction using flat electrochemical cells having a sealed configuration.
Still another advantage of the present invention relates to providing a bipolar battery construction wherein nickel-hydride electrodes may be used.
These and still other advantages and benefits may be achieved by making a bipolar electrochemical battery comprising:
a stack of at least two electrochemical cells electrically arranged in series with the positive face of each cell contacting the negative face of an adjacent cell, wherein each of the cells comprises
(a) a negative electrode;
(b) a positive electrode;
(c) a separator between the electrodes, wherein the separator contains an electrolyte;
(d) a first electrically conductive lamination comprising a first inner metal layer and a first polymeric outer layer, said first polymeric outer layer having at least one perforation therein to expose the first inner metal layer, said first electrically conductive lamination being in electrical contact with the outer face of the negative electrode; and
(e) a second electrically conductive lamination comprising a second inner metal layer and a second polymeric outer layer, said second polymeric outer layer having at least one perforation therein to expose the second inner metal layer, said second electrically conductive lamination being in electrical contact with the outer face of the positive electrode; wherein the first and second laminations are sealed peripherally to each other to form an enclosure including the electrodes, the separator and the electrolyte.
The present invention further relates to an electrochemical wafer cell comprising:
(a) a negative electrode;
(b) a positive electrode;
(c) a separator between the electrodes, wherein the separator contains an electrolyte;
(d) a first electrically conductive lamination comprising a first inner metal layer and a first polymeric outer layer, said first polymeric outer layer having at least one perforation therein to expose the first inner metal layer, said first electrically conductive lamination being in electrical contact with the outer face of the negative electrode; and
(e) a second electrically conductive lamination comprising a second inner metal layer and a second polymeric outer layer, said second polymeric outer layer having at least one perforation therein to expose the second inner metal layer, said second electrically conductive lamination being in electrical contact with the outer face of the positive electrode; wherein the first and second laminations are sealed peripherally to each other to form an enclosure including the electrodes, the separator and the electrolyte.
The present invention still further relates to an assembly for containing contents of a wafer cell, comprising:
(a) a first electrically conductive lamination comprising a first inner metal layer and a first polymeric outer layer, said first polymeric outer layer having at least one perforation therein to expose the first inner metal layer, said first electrically conductive lamination capable of being in electrical contact with a negative electrode; and
(b) a second electrically conductive lamination comprising a second inner metal layer and a second polymeric outer layer, said second polymeric outer layer having at least one perforation therein to expose the second inner metal layer, said second electrically conductive lamination capable being in electrical contact with a positive electrode, wherein the first and second laminations are capable of being sealed peripherally to each other to form an assembly for containing the contents of a wafer cell.
A further advantage of the present invention relates to enhanced conduction through the cell and/or adjacent cells due to the ease through which current may flow through the metal layers of the laminations exposed by the perforations.
Further advantages of this invention will be apparent to those skilled in the art from the following detailed description of the disclosed bipolar electrochemical batteries and methods for producing the bipolar electrochemical batteries, and of the wafer cells used therein.